The subject matter disclosed herein relates generally to cryogenically cooled magnetic resonance imaging (MRI) systems, and more particularly to systems and methods for providing a thermal shield for the MRI systems.
In superconducting coil MRI systems, the coils forming the superconducting magnets are cryogenically cooled using a helium vessel. The cryogen cooling system of some of these MRI systems include a coldhead within a coldhead sleeve that operates to recondense vaporized cryogen to continually cool the superconducting magnet coils during system operation. Additionally, a thermal shield may be provided, which is typically positioned within the vacuum vessel between vacuum vessel and the helium vessel.
Conventional thermal shields have to be formed as thick metal structures from a high thermal conducting material, such as aluminum, to provide the necessary thermal conduction. However, the thickness of these structures not only have a higher mass, but require longer time periods to cool down, as well as being electrically conducting. Additionally, the structures have higher vibration induced field instability due to high conductance.
Also, when the coldhead is off, for example, during transportation of the MRI system, power off of the MRI system during normal operation, or coldhead failure, the coldhead sleeve is heated due to contact between the coldhead and the coldhead sleeve. During this time, the coldhead sleeve acts like a heat sink (or heat source) and applies heat to the MRI system, including to a thermal shield and the helium vessel of the MRI system. In this condition with the coldhead sleeve acting like a heat sink and heating up the thermal shield and helium vessel, helium inside the helium vessel boils off. Thus, helium from the helium vessel is lost and must be replaced, which results in added cost and system maintenance as there is no path for cooling of the thermal shield.
Moreover, making the structures thinner would not provide the needed thermal conduction. Accordingly, higher temperature gradients and a greater likelihood of cracking or breaking of the structure would result.